Runoff Lesson Objectives

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RunoffLesson Objectives

• 1. Understand the storm and watershed
  characteristics that effect the rainfall-runoff
  relationship.
• 2. Understand the concepts behind the SCS Curve
  Number Method for predicting the peak runoff rate
  from a watershed.

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RunoffLesson Objectives

• 3. Be able to use the SCS Curve Number Method to
  predict peak runoff rate for a watershed.

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The Hydrologic Cycle
Solar Energy
WIND MOVEMENT
CONDENSATION
EVAPOTRANSPIRATION
PRECIPITATION
EVAPORATION
RUNOFF
STREAMS
LAKES STREAMS
GROUND WATER
INFILTRATION
OCEAN
DEEP PERCOLATION
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RUNOFF HYDROGRAPH
Peak Rate
Q
Flow Rate (Volume/Time)
Area under the curve represents total
runoff volume.
TIME SINCE START OF STORM
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RAINFALL FACTORS AFFECTING RUNOFF

• STORM INTENSITY
• STORM DURATION
• DISTRIBUTION IN TIME
• DISTRIBUTION OVER THE WATERSHED

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WATERSHED FACTORS AFFECTING RUNOFF

• SIZE
• SHAPE
• TOPOGRAPHY (Average land slope)
• GEOLOGY (Soil and bedrock)
• SURFACE COVER
• INITIAL SOIL MOISTURE

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SCS RUNOFF METHOD
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RUNOFF CURVE NUMBERSTable 5.3All good Stand
Contoured
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TIME OF CONCENTRATION

• The time required for water to flow from the most
  remote point of the area to the outlet.
• A measure of how watershed shape and topography
  affect the peak rate of runoff
• Depends on stream flow velocity as well as
  distance from outlet

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Affect of Watershed ShapeTwo Watersheds with
same Area, Slope and Soils
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